Paternity and relatedness in a polyandrous nonhuman primate: testing adaptive hypotheses of male reproductive cooperation

Male reproductive cooperation is rare in nature: expectations from evolutionary theory suggest that males should be competitors for reproductive opportunities and provide little parental care. Counter to this expectation, in cooperatively polyandrous mating systems, multiple adult males mate with a single breeding female and subsequently cooperate to rear her young. This raises the question of the fitness benefits of parental care, as males may be providing care to offspring that are not their own. Here, I use genetic and demographic data from a multiyear field study of Geoffroy’s tamarin, Saguinus geoffroyi, to test predictions of the indirect and direct fitness benefits hypotheses. I found that polyandrous males within a group were related at levels consistent with filial or fraternal relationships (r = 0.36–0.44) and could also share paternity (40% of groups had >2 male sires). Sharing of paternity occurred both within litters and over multiyear associations that remained stable throughout the study period of 2–3 years. However, remaining groups had a single sire, perhaps owing to the ability of males to prevail in sperm competition. These results suggest the joint role of indirect and direct fitness benefits in male–male cooperation in tamarins, in contrast to other cooperatively polyandrous species. The high average relatedness of polyandrous males may be consistent with fraternal cooperative polyandry in this species, as observed in certain human societies, providing a novel example that should enhance comparative studies of the evolution of male–male cooperation in reproduction.